Chip, a post titled "old sunn 115 cab info" in the Q&A section sheds some light on this. It was pretty long thread, but in one of the posts EdBass (7-5-07) identified the 29x24x11.5 cab as a Sonaro cabinet, originally fitted with a Sunn Bass Transducer. In 1969 the Sonaro was a piggyback rig, as Loudthud mentions, and not a one-piece combo as it had been previously.
According to Ed's same post, the 38x24x11.5 cab was for the Sentura I (as you suspected), and originally came with a JBL D15S.
I have two of the 115RH cabs, both outfitted with JBL E-140 speakers. They are great individually or together.
Unfortunately, I missed the Nashville show, too. Mike pushed the date back a month or so beforehand and I couldn't go on the new weekend. I'll tell him we talked here, though!
Carter